1. FIELD OF THE INVENTION
The present invention relates to a thermal flow sensor which measures the flow rate of fluid by using a thermal resistance element.
2. DESCRIPTION OF THE RELATED ART
A thermal flow sensor is disclosed, for example, in Japanese Utility Mode Laid-Open No. 61-108930. This thermal flow sensor is constructed in such a manner that a thermal resistance element is disposed in a fluid passage, and the flow rate of the fluid is measured according to the temperature conditions of the thermal resistance element.
FIG. 3 is a view showing the structure of such a flow sensor. In FIG. 3, a rectification net 2 is positioned on the upstream side of a cylindrical conduit 1 which guides a fluid to be measured, this rectification net 2 being used for rectifying the flow of the fluid to be measured flowing inside the cylindrical conduit 1. As shown in FIG. 4, the rectification net 2 is made by combining wires 2a into a lattice-like structure. A thermal resistance element 3 which is cooled by the fluid is disposed on the downstream side of the rectification net 2. The thermal resistance element 3 is fed with control electric power from an electronic circuit 4. This electronic circuit 4 is composed of resistors 5, 6, 7, a differential amplifier 8, a transistor 9, and a power supply 10. The resistors 5, 6, 7 form a Wheatstone bridge circuit together with the thermal resistance element 3. The differential amplifier 8 is connected to the middle point of the Wheatstone bridge. The conductivity of the transistor 9 is controlled by the differential amplifier 8. The power supply 10 feeds the Wheatstone bridge circuit with operational electric power through the transistor 9.
With the above structure, the values of the resistors 5, 6, 7 are so set or chosen that the thermal resistance element 3 reaches a predetermined temperature. When a fluid is fed into the conduit 1, the resistance element 3 is cooled as the fluid flows. The degree of this cooling is directly proportional to the flow rate of the fluid. On the other hand, since the resistance values of the thermal resistance element 3 vary, it is possible to measure output in accordance with the flow rate at a node "A" where the thermal resistance element 3 is connected to the resistor 6. This measurement is made possible by controlling the transistor 9 through the differential amplifier 8. It is thus possible to calculate the mass flow rate of the fluid.
Furthermore, in the above situation when the flow of a fluid acting on the thermal resistance element 3 is disturbed, it is difficult to accurately measure the flow rate of the fluid. The fluid can be stabilized by placing the rectification net 2 upstream from the thermal resistance element 3.
Now, in such a thermal flow sensor, when the rectification net 2 is positioned close to the thermal resistance element 3, the flow velocity of a fluid is unevenly distributed. In other words, the fluid flows slowly downstream of the wires 2a which make up the rectification net 2, whereas the fluid flows fast at the openings of the wires 2a. As expressed in the following equation, it is known that the quantity of heat "Q" which is absorbed by the fluid from the surface of the thermal resistance element 3 is directly proportional to the surface area of the thermal resistance element 3 exposed to the fluid: EQU Q=.alpha. (t.sub.0 -t.sub.1) S,
where .alpha. is the heat transfer rate of the entire thermal resistance element 3;
t.sub.0 is the surface temperature of the thermal resistance element 3;
t.sub.1 is the temperature of the fluid; and
S is the surface area of the thermal resistance element 3.
Thus, as shown in FIGS. 4 and 5, the area of the thermal resistance element 3 facing the openings between the wires 2a varies according to the placement of the thermal resistance element 3 and the rectification net 2. This fact indicates that the effective surface area "S" of the thermal resistance element 3 varies. As a result, the output characteristics of the thermal flow sensor also vary according to the attachment relationships between the rectification net 2 and the thermal resistance element 3.
Thus, in the conventional thermal flow sensor, the thermal resistance element 3 must be placed farther away than a fixed distance (L) where the problem of uneven flow is solved. This fact is an obstacle to the miniaturization of the thermal flow sensor.
For the thermal resistance elements 3 as shown in FIGS. 6 and 7, it is confirmed from experiments that the fixed distance (L) must be at least 20 times the width (W) or diameter (d) of the thermal resistance elements 3. In this embodiment, the sizes of the thermal resistance element 3 are as follows:
length (l) .gtoreq.width (W) .gtoreq.thickness (t),
or length (l) .gtoreq.diameter (d)